The present invention relates to a process and apparatus for shutdown and control of a nuclear reactor, and more especially to a process and apparatus for the shutdown and control of a gas-cooled nuclear reactor, in which the coolant gas is passed through from the top to the bottom of the core which is formed by a pile of spherical fuel elements and the fuel elements have reached the desired final burn-up afer one pass through the pile, and in which the core is surrounded by a reflector, consisting of a top reflector, a cylindrical side reflector and a bottom reflector, and which also embodies devices for afterheat removal.
There is known a gas-cooled nuclear reactor having a core consisting of a pile of spherical fuel elements which for control and adjustment processes is equipped with core absorber rods and with movable absorber rods in the side reflector (hereinafter also called core rods and reflector rods, respectively). This reactor is known as high temperature reactor THTR-300. In this reactor type, the core rods and the reflector rods are used for start-up, i.e., for the control of renewed start-up of the reactor after shutdowns. The core rods are provided for partial or rapid shutdown (scram) and for total or long-term shutdown. The manner in which the reactor is abruptly changed to subcritical status by means of a rapid introduction of negative reactivity is designated as partial or rapid shutdown of the reactor (scram).
When a rapid shutdown becomes necessary, in the event of a possible malfunction, the core rods are inserted to a predetermined position at such a speed that the reaction immediately reaches a subcritical status and can be maintained in this condition over an extended period of time. During this time, malfunctions can be analyzed and removed. It is very desirable that such analysis and removal of a malfunction take place in as short a time period as possible, so that the reactor may be returned to critical status and full power within the period of about one hour, by way of a so-called hot start. However, such a hot start is possible only when the Xenon-135 concentration, which increases during shutdown of the reactor, has not yet reached too high a value by the time the power of the critical reactor is increased again. Otherwise, a new start-up of the reactor is possible only after approximately 24 hours, when the Xenon-135 concentration has subsided through decay.
The time span over which the reactor can be maintained in a subcritical condition through this operating procedure is determined by the value of the shutdown reactivity which is introduced and by the devices for the afterheat removal which are used in the reactor. If it is not possible to repair the damage within a short period of time, a long-term shutdown will subsequently be necessary. For this purpose, the core rods and their adjustment drives are designed so that in the fully inserted position of the rods, they can supply to a reactor the negative reactivity which is necessary for a long-term shutdown.
Thus, the rapid and long-term shutdown will be carried out in the known reactor by one and the same shutdown system, which is formed for both shutdown procedures by the core rods. Reflector rods may additionally be employed in connection with the rapid shutdown, but only to the extent to which they are available and are not already inserted for compensation of excess reactivity resulting from a rapid load adjustment maneuver. They are not included in the shutdown system.
Adjustment of the reactor (as opposed to shutdown) is accomplished by means of the reflector rods which are designed for this purpose. They serve both for the fine adjustment of a predetermined performance level or condition and for any rapid load adjustments involving adjustment of the reactor within predetermined power values defined by the maximum possible power output and a lower power limit. The known reactor has power limits of 100% and 40%. If the power is reduced for example to 40%, the lowered neutron flux results in a reduced capture of Xenon-135, while the Xenon-135 production through iodine disintegration initially continues unabated, so that, after about five hours, a maximum Xenon-135 poisoning is reached. Withdrawal of nearly all inserted reflector rods makes it possible to maintain the reactor in critical status. If the reactor is subsequently operated at 40% partial load for an extended period of time, the Xenon-135 concentration returns to a value which is slightly below the value at full load, because the production of the fission product iodine has adapted itself to the reduced power level, and some of the reflector rods must be again inserted to compensate the excess reactivity. If, after extended partial load, the reactor is rapidly returned to full load, a drop in the Xenon-135 concentration takes place because of the increased capture by neutrons. An over-critical condition of the reactor is prevented by insertion of nearly all reflector rods.
In order to meet the high safety requirements, demanded in the reactor technology, there is provided in the known reactor type a further shutdown system (emergency shutdown system). In this system an absorbing agent, for example, a neutron-absorptive gas such as borontrifluoride is fed into the reactor, or small absorber balls are inserted. The introduction of the absorbing agent is started by hand if the shutdown system, formed by the core absorber rods, is not fully functionable for any reason. In the event of failure of the first shutdown system, the emergency shutdown system automatically comes into operation, in any case. This results in the disadvantage that a longer shutdown of the reactor is necessary, since the absorbing agent which is fed into the reactor core must first be removed from the core.
In another known nuclear reactor utilizing a pile of spherical fuel elements, the arrangement for power adjustment and for control is constructed in such a manner that the absorber rods are insertable through the top reflector up to a predetermined insertion depth into the chamber enclosed within and defined by the reflector (German Offenlegungsschrift No. 2,353,653). This arrangement comprises a portion of the rods for shutdown and a portion of the rods for power adjustment, whereby the portion serving for shutdown of the reactor consists of absorber rods which are insertable into the pile, and the portion serving for the power adjustment is formed by absorber rods which are slidable within the wall of the top reflector and within the space formed between the pile and the top reflector. Thus, these absorber rods are not inserted into the pile, so that no forces opposed to the movement direction of the absorber rods need be overcome during the power adjustment, as is the case when using absorber rods which are inserted directly into the pile. It has been shown that the absorber rods inserted into the space above the pile for adjustment of the nuclear reactor are especially efficient, when the known reactor is operated in such a manner that the combustion elements pass the pile only one time. This is because the axial neutron flux profile in the reactor core shows a maximum in the upper third of the core. That part of the adjustment and control arrangement serving for the shutdown of the reactor can also be used for purposes of adjustment. The thus-affected absorber rods are then formed so that they are also slidable in the wall of the top reflector and into the space between the top reflector and the fuel element pile. This known nuclear reactor has the disadvantage that it is provided with one shutdown system only, so that another shutdown system (emergency shutdown) cannot be put into operation in case of a failure of that part of the adjustment and control arrangement serving for the shutdown. In a nuclear reactor with a single pass of the fuel elements, there is another disadvantage that the side reflector is charged by a neutron dose approximately twice as high as that occurring by multi-pass of the fuel elements. If the side reflector cannot be removed regularly, special measures for the protection of the side reflector must be taken.
Another known nuclear reactor with spherical fuel elements embodies an adjustment and control device which also comprises absorber rods which are insertable directly into the fuel element pile plus adjustment rods arranged in the side reflector (German Offenlegungsschrift No. 2,123,894). However, the adjustment rods are essentially moved only within the top reflector, i.e., they are not inserted into the space between the top reflector and the fuel element pile. Also in this nuclear reactor, there exists only one shutdown system, and the side reflector is exposed to high loads by fast neutrons, as a result of the high power density in the upper third of the reactor core.
Furthermore, reference is made to a nuclear reactor characterized by a single pass of the spherical fuel elements in which are provided measures for the protection of the upper part of the side reflector as well as the top reflector (German Offenlegungsschrift No. 2,347,817). These measures consist in providing agents within the wall of the side reflector which absorb neutrons or decrease the neutron speed. These agents can be rod-shaped and can be arranged in corresponding recesses. It is not known in which manner the shutdown and the adjustment are handled in this nuclear reactor.